The present invention relates generally to a method for a field compensation of an electronic compass according to claim 1, to a handheld observation device with a field compensatable electronic geomagnetic compass functionality according to claim 6, to a digital magnetic compass module according to claim 12 and to a computer program product.
The present invention relates to a setup and method for a compensation of an electronic magnetic compass, in particular a Digital Magnetic Compass (DMC). Such an electronic compass can for example be integrated into a surveillance or observation device for providing north-referenced azimuthal orientation information of a direction or target aimed by a Line of Sight (LOS) of the device, for example north referenced azimuth information of an optical axis of an observation device. Such an observation device can for example be a sighting device such as monocular or a binocular, night vision equipment, telescopic sight, periscope, etc. with a fully optical or partially electronic observation path. Beside the DMC discussed in this application, such an observation device can be equipped with or connected to additional sensors and measurement equipment such as goniometers for determining elevation and/or bank information of the LOS, an electronic distance meter (EDM) such as a Laser Range Finder (LRF) for determining a distance to a target aimed with the LOS, navigation-, mapping- and location-equipment like a GNNS-Receiver (GPS, GLONASS, GALILEO, . . . ), wired or wireless remote controllers and/or displays, image or video capturing units, thermographic cameras, Range Imaging cameras (RIM), etc. Examples for such observation devices are the Vectronix Moskito, Vector, PLRF or JIM LR.
DMCs (as well as other sorts of magnetic compasses) are influenced in their heading determination by ferromagnetic and/or electromagnetic objects interfering with the geomagnetic field of the planet. Ferrous or permanent magnetic objects as well as electrical currents result in a disturbance of the geomagnetic field at the magnetic sensing unit which is used to determine the magnetic fields for magnetic compassing. Those disturbances can falsify the determined north reference or even render a determination of a magnetic north reference impossible. Therefore, such magnetic influences need to be compensated from the uncompensated, raw magnetic field measurements by the magnetic sensors of the compass in order to determine a magnetic compass bearing.
The methods for a possible further determination of a true bearing or grid north by taking into account the local declination are known in the art, as well as the therefore used reference models like the IGRF or WMM. The present invention only explains the basis for latter, namely a correct determination of a true bearing referenced to the local geomagnetic field—giving magnetic compass north—which will be also referenced as azimuth, for example given in units of degrees, degrees west/east, mils or grads. Nevertheless, the abovementioned further corrections of the magnetic compassing, for example to gain the direction of the magnetic north pole, the geographic north pole or another derivable geographically referenced azimuth direction are not excluded, although they will not be discussed here in detail.
If a digital magnetic compass is for example built into an observation device as described above, a magnetic compensation is needed to remove the influence of magnetic parts within the device itself or attached to the device, like e.g. a rifle attached to a telescopic sight to be used as observation device with the electronic compass. Those magnetic parts are in particular ferromagnetic parts, electronic equipment and/or batteries as sources of magnetic fields by therein occurring electric currents. The parts can be located within the device, like in an integrated LRF, display, camera, computation means, electric actuators (with or without permanent magnets), components made of iron, screws, bolts, etc. or they can be close to and moved with the device, like glasses or a helmet worn by the user of the observation device, a part of a vehicle where the observation device is attached to or other parts influencing magnetic fields which have a (relatively) defined arrangement with respect to the magnetic sensors of the compass.
To achieve such a compensation, the article “A NON-LINEAR, TWO-STEP ESTIMATION ALGORITHM FOR CALIBRATING SOLID-STATE STRAPDOWN MAGNETOMETERS” by D. Gebre-Egziabher, G. H. Elkaim, J. D. Powell and B. W. Parkinson from the Department of Aeronautics and Astronautics at Stanford University, published in Proceedings of the International Conference on Integrated Navigation System, St. Petersburg Russia, May 28-30, 2001, pp. 290-297, explains the principles of magnetic compass compensation and an example of method for solving such.
U.S. Pat. No. 6,009,629 discloses a method to determine the direction of the Earth's magnetic field in the case of interference by magnetic material built into equipment. For this purpose, an electronic magnetic compass mounted on the equipment is brought into N different spatial positions, where magnetic field and inclination angles measured in a magnetic compensation procedure. The thereby gathered data allows the use of a mathematical algorithm to determine the true earth field and remove the disturbances by therein presented magnetic compensation formulas.
Electronic magnetic compasses are for example available as pre built modules like the Vectronix DMC-SX, which already provides the basic routines and algorithms for the execution of such a compensation in their firmware. Those digital compass modules can be built into an observation device, which provides additional functions like day and night observation, laser range-finding, as discussed above.
For gathering data for the compensation, a prescribed pattern of geometric compensation-measurement orientations, which provide sufficient information for the determination of the magnetic compensation parameters, is desired. Although there are many possible embodiments for such patterns for gathering data needed for the compensation calculation, there are certain patterns which are advantageous in view of the acquired data, number and sequence of points, and also of the achievability of those orientations by nature of the device and setup comprising the compass, etc. Depending on the ambient conditions, there can be certain optimal patterns as discrete sets of multiple orientations of the compass respectively the device comprising the compass, which are preferable for a certain compensation task.
As the compensation patterns can be relatively complicated for an inexperienced user (see e.g. FIG. 1 and FIG. 2 discussed below), it is advantageous to guide the user to the positions, which are described by their relative azimuth, elevation and bank, not only by paper instructions but with the help of a display and software of the observation device. Thereby, the user is interactively guided by the user interface to aim the device into a set of desired geometric orientations (azimuth, pitch, roll) for the compensation measurements.
The orientations of the device do not need to be aimed with high precision. For example, a range of roughly about ±20° in azimuth and ±10° in elevation and bank are in general sufficient to gather reasonable compensation data. A typical compensation procedure covering all the desired orientation can for example be executed in the order of about one minute.
For the purpose of user guidance for the compensation data acquisition, the angles provided by the compass itself are used to guide the user to the desired orientations of the set. This is no problem for elevation and bank as those can be determined according to the direction of gravity by accelerometers with more than reasonable accuracy for the user guidance, but it can become too imprecise or even impossible for azimuth in case of severe magnetic disturbances, for example due to magnetic batteries mounted too closely to the compass due to the smallness of the device.
When the magnetic field is too disturbed to provide even the low accuracy azimuth needed for the guidance, such a built-in user guidance based on the uncompensated internal magnetic compass no longer works. This problem is getting worse in a growing amount of cases, in particular caused by smaller sizes of the devices, stronger magnetism of batteries and also bad construction (battery too close to DMC) of some devices. Also, the increased electrification of the equipment and more and more soft-, hard- and/or electromagnetic items in the vicinity of the magnetic compass sensors increases those problems.
Even if the patterns to be subsequently aimed with the device for compensation are tried to be kept so simple that a user should also be able to execute them without guidance, once guidance is offered by the device as standard routine or in particular if such a compensation guidance routine is already started and/or at least partially executed, it should be executable and possible to complete in any instance. A simple “compensation failed”, “compensation not possible” or “guidance impossible” message or a termination without a result is not acceptable.
For devices which are used stationed on an earth fixed support, e.g. on a tripod, with a horizontal rotatable platform equipped with a goniometer, an azimuth encoder can be used for determining the device's orientation for achieving the desired compensation pattern. Nevertheless, the requirement for such goniometers and a stationary base results in a more complex system is not applicable in many used cases, e.g. for a handheld device.
Other known compensation solutions, as for example used for an electronic compass in mobile devices such as a smartphone or a satnav, only allow a quick and dirty compensation of some of the influences. For example to spin the device around its horizontal axis or to move the device continuously in a Möbius strip for calibration cannot compensate all the influences in a decent manner to achieve the desired accuracy in a range of a few degrees or less, as e.g. also explained in abovementioned U.S. Pat. No. 6,009,929. Furthermore, such a movements along a three dimensional 8-shaped path are quite difficult to explain and to execute for an inexperienced user or when the device is attached to some object. The approach presented here is different to those approaches which can only achieve a partial compensation with much lower accuracy and reliability. In the presently discussed approach accuracies in the region of a few degrees, preferably below 0.5° or less are desired, in particular for observation devices. Such accuracies are far below the ones required for car navigation systems or for determining the orientation of a mobile phone.